Method of manufacturing a pole face for a permanent magnet MRI system with laminated structure

ABSTRACT

A method of manufacturing a magnetic resonance imaging (MRI) device is provided including providing at least one magnet positioned between a keeper device and a yoke, the keeper device being positioned at a pole region of the at least one magnet, positioning at least one pole device at the pole region of the at least one magnet, and removing the keeper device from the pole region to allow the at least one pole device to be positioned at the pole region of the at least one magnet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/248,170 nowU.S. Pat. No. 6,825,666, entitled “POLE FACE FOR PERMANENT MAGNET MRIWITH LAMINATED STRUCTURE”, filed on Dec. 23, 2002.

BACKGROUND OF THE INVENTION

The present invention relates generally to magnetic resonance imaging(MRI) devices, and more particularly to MRI devices with a removablekeeper plate.

MRI devices are widely used in the medical community as a diagnostictool for imaging items such as tissue and bone structures. As described,for example, in U.S. Pat. No. 5,680,086 (which is incorporated byreference herein in its entirety), a conventional MRI device may includeopposing pole pieces which define between them an imaging volume for theitem to be imaged, the pole pieces having pole plates which arefabricated from wound high permeability soft magnetic material, and/orlaminated members. One conventional MRI manufacturing technique isdescribed in European Patent Application EP 0 978 727 A2 (“EP '727”hereafter), filed on Jul. 22, 1999, which is incorporated by referenceherein in its entirety.

In conventional MRI device manufacturing techniques, a yoke is assembledbetween two ends of the MRI device, and two pole pieces are positionedthereon with a gap provided underneath the pole piece at each end forthe MRI magnet. The MRI magnet is then formed by pushing individualpre-magnetized permanent magnet blocks in place under the pole pieces oneach end of the MRI device. Once all of the pre-magnetized permanentmagnet blocks are positioned and secured in place, the pole pieces arelowered to their ultimate position and secured in place.

An MRI device manufactured by the aforementioned manufacturingtechnique, however, suffers from inclusion of a backing plate used toreinforce the pole pieces during the manufacturing process.Specifically, in the course of pushing the pre-magnetized permanentmagnet blocks into place, the pole pieces must endure large bendingforces due to the moving pre-magnetized permanent magnet blocks. Abacking plate is used to overcome the bending forces by stiffening thepole pieces. The backing plate, however, affects the magnetic fieldgenerated by the permanent magnet block array. In highly precise MRIdevices, this can lead to a degradation in the detection properties ofthe MRI device.

Thus, a need exists for a method of manufacturing an MRI device free ofa backing plate in a pole face of the pole piece, and/or a method ofmanufacturing an MRI device with the pole pieces being subjected to aweaker and/or channeled magnetic fields during assembly.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed at reducing or eliminating one or moreof the problems set forth above, and other problems found within theprior art.

According to one aspect of the present invention, a method ofmanufacturing a magnetic resonance imaging (MRI) device is providedcomprising the steps of providing at least one magnet positioned betweena keeper device and a yoke, the keeper device being positioned at a poleregion of the at least one magnet, positioning at least one pole deviceat the pole region of the at least one magnet, and removing the keeperdevice from the pole region to allow the at least one pole device to bepositioned at the pole region of the at least one magnet.

According to another aspect of the present invention, a magneticresonance imaging (MRI) device including a yoke and at least one magnetaligned with the yoke is provided, the at least one magnet generating amagnetic field within the MRI device, the MRI device comprising at leastone pole device for shaping the magnetic field generated by the at leastone magnet, the at least one pole device being positioned at a poleregion of the at least one magnet and being free of a solid metalbacking plate in a pole face of the at least one pole device.

According to another aspect of the present invention, a magneticresonance imaging (MRI) device is provided comprising means forgenerating a magnetic field, means for shaping the magnetic field, andmeans for positioning the means for generating a magnetic field at apole region of the means for shaping the magnetic field.

According to another aspect of the present invention, a method ofreconfiguring a magnetic resonance imaging (MRI) device is providedcomprising the steps of positioning a keeper device at a pole region ofat least one magnet, moving the at least one magnet from an initialposition to a reconfigured position, and removing the keeper device fromthe pole region of the at least one magnet to allow at least one poledevice to be positioned at the pole region of the at least one magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a MRI device undergoing an assembly stepaccording to a preferred embodiment of the present invention.

FIG. 2 is a block diagram of the MRI device of FIG. 1 undergoing adifferent assembly step according to a preferred embodiment of thepresent invention.

FIG. 3 is a block diagram of an assembled MRI device according to apreferred embodiment of the present invention.

FIG. 4 is a flow chart of a method of manufacturing a MRI deviceaccording to a preferred embodiment of the present invention.

FIG. 5 is a flow chart of a method of reconfiguring a MRI deviceaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to presently preferred embodimentsof the present invention. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

A MRI device 100 undergoing assembly at one end thereof according to afirst embodiment of the present invention is shown in the block diagramof FIG. 1. For purposes of illustration only, the following descriptionand the accompanying Figures only describe the manufacture of one end ofthe MRI device 100. It should be appreciated, however, that the oppositeend of the MRI device 100 can be manufactured from the same (or similar)technique, simultaneously with, before, or after the end depicted isassembled.

The MRI device 100 includes an iron yoke 110 and a keeper plate 150.Preferably, the iron yoke 110 is first assembled on the MRI device 100,and then the keeper plate 150 is temporarily positioned thereon. Oncethe keeper plate 150 is secured in place, a plurality of permanentmagnet blocks 120 are slid into place beneath the keeper plate 150(collectively the assembled “MRI magnet” 125 in FIG. 2) as indicated bythe arrows pointing from right to left. The assembled MRI magnet 125 canbe made of any one of a number of conventional magnetic materials, andformed in a conventional manufacturing process as is well known in theart (see, for example, U.S. Pat. No. 4,770,723, which is incorporatedherein by reference in its entirety). The assembled MRI magnet 125 isarranged on (or near) iron yoke 110 in such a way as to generate amagnetic field within the MRI device 100 for imaging an item. Oneexemplary arrangement is described in U.S. application Ser. No.09/824,245 filed on Apr. 3, 2001, which is incorporated by referenceherein in its entirety.

As shown in FIG. 2, once the MRI magnet 125 is assembled, the keeperplate 150 is “swapped” with pole face laminate 130. The keeper plate 150is adapted to channel at least a portion of the magnetic field generatedby the assembled MRI magnet 125 while the pole face laminate 130 ispositioned. In FIGS. 1 and 2, the keeper plate 150 is shown positionedat the pole regions near the sides of the assembled MRI magnet 125.Hence, the pole face laminate 130 is subjected to a weaker magneticfield, which makes it easier to properly align and position the poleface laminate 130, and also eliminates the need for a solid metalbacking plate used to position and protect (e.g., via sliding) the poleface laminate 130 during assembly. Thus, in the present invention, thepole face laminate 130 can be used without a backing plate.Alternatively and less preferably, a solid metal backing plate may beused in conjunction with the keeper plate 150.

It should be appreciated that the pole face laminate 130 is positionedin such a way as to shape and focus the magnetic field generated by theassembled MRI magnet 125 in the item to be imaged (e.g., by drawingmagnetic flux generated by the assembled MRI magnet 125). Preferably,the pole face laminate 130 is thus positioned between the assembled MRImagnet 125 and the “hypothetical” item to be imaged. The pole facelaminate 130 can be made as described, for example, in U.S. applicationSer. No. 09/198,507 filed on Nov. 24, 1998 which is incorporated byreference herein in its entirety.

As noted above, the keeper plate 150 is preferably removable, such thatthe keeper plate 150 can be gradually removed during simultaneouspositioning of the pole face laminate 130. Swapping the keeper plate 150with the pole face laminate 130 provides for easier removal of thekeeper plate 150 and placement of the pole face laminate 130, as thepole face laminate 130 and the keeper plate 150 are only partiallysubjected to the magnetic field generated by the assembled MRI magnet125 by sharing the magnetic flux load with the other. An MRI device 100manufactured by the aforementioned process is shown in the block diagramof FIG. 3. As shown, the MRI device 100 includes yoke 110, two pole facelaminates 130, and two assembled MRI magnets 125. The MRI device 100 isfree, however, from a backing plate attached to each of the pole facelaminates 130. By eliminating the backing plate, the cost and complexityof the MRI device 100 and machinery necessary to assemble it can bereduced. Furthermore, elimination of the backing plate eliminates anymagnetic interference previously caused by the backing plate, resultingin a more uniform (and predictable) magnetic field for imaging the itemto be imaged.

A method of manufacturing an MRI device 100 according to an embodimentof the present invention will now be described in reference to theflowchart of FIG. 4. It should be appreciated that the order of themethod steps may vary and/or be combined, as would be readily apparentto one of ordinary skill in the art after reading this disclosure.Furthermore, additional method steps may be provided as needed, such ascleaning steps, gluing steps, etc. In other words, the followingdescription is not an exhaustive list of every method step or orderthereof within the scope of the present invention.

In step 410, an iron yoke is positioned in an MRI device. A magnetickeeper plate is then positioned substantially above the iron yoke instep 420, providing a gap between the magnetic keeper plate and the ironyoke for a MRI magnet. A plurality of permanent magnet blocks(collectively the assembled “MRI magnet”) are then arranged in the gapbetween the magnetic keeper plate and the iron yoke in step 430 in sucha way as to generate a magnetic field in an item to be imaged. Thepermanent magnet blocks may be formed in a conventional manner aspreviously described. Preferably, the magnetic keeper plate ispositioned in step 420 prior to assembling the MRI magnet on the ironyoke in step 430 in order to channel the magnetic field generated by theMRI magnet as it is being assembled.

After the MRI magnet is assembled on the iron yoke in step 430, thekeeper plate will be positioned at the pole region of the magnet. Thepole face laminates are then positioned in step 440. Preferably, thepole face laminates are positioned in step 440 simultaneously whileremoving the keeper plate in step 450 (e.g., by sliding the keeper plateout of the device), thereby “swapping” the pole face laminates for thekeeper plate. This allows the pole face laminates to be easilypositioned due to the magnetic field being partially channeled by thekeeper plate as it is removed. Once the pole face laminates arepositioned at the pole regions in step 440, the keeper plate is thencompletely removed from the device in step 450, and can be used toassemble another MRI device (if desired).

One advantage of the aforementioned removable keeper plate is theability to reconfigure an MRI device previously assembled using theaforementioned manufacturing process (see FIG. 5). To reconfigure theMRI device, the keeper plate is moved back into a pole region positionin step 510. Preferably, the keeper plate is moved back into position instep 510 while simultaneously removing the pole face laminates in step520. The pole face laminates can then be exchanged for new orreconfigured pole face laminates, the MRI magnet can be adjusted (e.g.,by adding, removing, or relocating permanent magnet blocks), etc. andthen repositioned in step 530. The keeper plate is then removed in step540, preferably while simultaneously performing step 530. Hence, thepresent invention provides for reconfigurable MRI devices, which couldnot be easily performed with conventional MRI devices.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto, and their equivalents.

1. A method of manufacturing a magnetic resonance imaging (MRI) device,comprising the steps of: positioning at least one magnet between amagnetic keeper plate and a yoke, the magnetic keeper plate beingpositioned at a pole region of the at least one magnet; and swappingsaid magnetic keeper plate with at least one pole device so as toposition said at least one pole device at the pole region of said atleast one magnet.
 2. The method of claim 1, wherein at least a portionof a magnetic field generated by said at least one magnet is channeledthrough said magnetic keeper plate while positioning said at least onepole device.
 3. The method of claim 1, wherein said providing stepcomprises: aligning said magnetic keeper plate with said yoke; andpositioning said at least one magnet in a gap between said magnetickeeper plate and said yoke.
 4. The method of claim 1, wherein saidswapping step comprises: sliding said magnetic keeper plate out of thepole region of said at least one magnet, wherein at least a portion of amagnetic field generated by said at least one magnet is channeledthrough said at least one pole device while sliding said magnetic keeperplate out of the pole region of said at least one magnet.
 5. The methodof claim 1, wherein said at least one pole device comprises: at leastone laminated pole plate.
 6. The method of claim 1, wherein said atleast one pole device comprises: a plurality of pole plates positionedadjacent to said magnetic keeper plate ice in said positioning step. 7.The method of claim 1, wherein said magnetic keeper plate comprises: atleast one solid pole plate.
 8. The method of claim 1, wherein the atleast one magnet comprises a permanent magnet.
 9. The method of claim 1,wherein said swapping step comprises gradually removing said magnetickeeper plate from the pole region of said at least one magnet whilesimultaneously positioning said at least one pole device at the poleregion of said at least one magnet.